In general, cellulose triacetate film is used as the transparent film of optical application or as the support for photographic material. The cellulose triacetate film is produced by solution film forming method.
In the solution film forming method, the polymer solution dissolved in organic solvent is cast from a die onto a casting band or a casting drum, and at the same time, a casting ribbon is made to touch closely to the casting band or the casting drum by the effect of a reduced pressure chamber which is provided in the neighborhood of the die.
The casting ribbon defines a liquid membrane between a discharge opening of the die and the surface of the casting band or the casting drum.
As an example of the solution film forming method, Japanese Patent KOKOKU 49-36946 proposes a casting method of liquid composition in which a reduced pressure chamber having two aspiration chambers in order to prevent an air companion make a liquid composition flow touch tight with the casting drum.
Moreover, Japanese Patent KOKOKU 62-38133 and Japanese Patent KOKOKU 63-57222 propose a uniform holding apparatus of the web that can stabilize edge beads by establishing two vacuum zones with an insulation wall.
Furthermore, Japanese Patent KOKAI HEI 5-86212 and so on propose the casting method which prevents development of an uneven surface drying defect by using a dope made by mixing a rich solvent and a poor solvent with specific rate.
However, as for the polymer resin film produced by the conventional solution film forming method, minute periodic lateral thickness unevenness occurs in a longitudinal direction of the film because the casting ribbon oscillates with a certain frequency caused by oscillation disturbance with wind pressure oscillation or machine oscillation such as air core oscillation in reduced pressure chamber, air core oscillation of a suction duct for pressure reduction, and the oscillation of the reduced pressure chamber transmitting from a blower connected to the suction duct.
In particular, in late years, as a thin film device technology of the liquid crystal display (referred to as LCD) progresses, a thin film technology is pursued in each element and a thin film technology is also demanded to protective coat for polarizing plate.
However, the leveling effect that is characteristic of a solution film forming method reduces by the thin film technology, and various kinds of thickness unevenness become distinguished.
By the way, the transparent film of optical application is applied an anti glare layer coating in order to give, for example, a hard coat or antireflection function. However, the coating unevenness that is caused by thickness unevenness in film base injures appearance value and functionality of the film of optical application triggering big problems on quality of the LCDs.
Moreover, similar problem occurs in emulsion coating on support for photographic material.
The object of the present invention is to solve the above-described problems and to provide a polymer resin film and the manufacturing method which does not cause any coating unevenness even when various functional layer are coated on the film produced by the a solution film forming method.